Commercial purity aluminum is the basis for the majority of the normal aluminum alloys. It is often used without any additions, such as in the production of utensils and foil. The production of metallic aluminum from alumina takes place in electrolytic cells or pots at a temperature of approximately 950.degree. C. Direct current is passed through a current-conducting salt bath in which alumina is dissolved. The bath consists of fused sodium aluminum fluoride (Na.sub.3 AlF.sub.6), commonly called cryolite, or a mixture of cryolite and other fluorides. Because alumina dissolves in the salt bath, the electrolysis takes place considerably under the melting point of alumina (about 2150.degree. C.). Aluminum fluoride, lithium fluoride, calcium fluoride or magnesium fluoride can be added to the bath in order to further lower the melting point and/or vapor pressure.
The typical electrolytic cell comprises a rectangular steel shell lined with refractory material as heat insulation, which in turn is lined with carbon. Carbon blocks in a bottom of the cell serve as the cathode. The cell holds the fuse salt electrolyte in which alumina is dissolved. Carbon anodes are suspended from above the cell and dip into the bath. When the cell is in operation, the bath is kept molten by the heat generated from the passage of electrical current. The surface is usually crusted over. Alumina is added to the bath as needed by breaking the crust. Under the influence of the electric current, aluminum metal is deposited at the negative pole and, therefore, collects at the bottom of the cell from where it is siphoned periodically. Oxygen is released at the anodes where it reacts with carbon, forming CO and CO.sub.2. Thus, the anodes and anode bars supporting the anodes in a conventional manner are consumed and must be replaced regularly. It is highly desirable to both prevent anode bars from being consumed rapidly, yet permit rapid restoration, refurbishment and/or replacement when so dictated.
Conventional aluminum reduction plants require a large amount of electrical energy, and by extending the life of electrodes or allowing inexpensive refurbishment thereof, electrical costs are maintained sufficiently low to assure the production of commercially competitive aluminum by increasing power efficiency and associated carbon anode efficiency, a reduction in the price of aluminum can be achieved and is, of course, compounded over time. Such savings involve a great deal of money (in the millions) and high anode efficiency is extremely advantageous under present Hall-Heroult cell processes using consumable anodes.